1. Field of the Invention
The present invention relates to a ringing tone signal detecting circuit, and mote particularly to a ringing tone signal detecting circuit for use in an echo-canceler digital subscriber line transmission system which employs paired metallic cables.
2. Description of the Related Art
In a digital subscriber line transmission system, a ringing signal called a tone signal is transmitted over a subscriber line to call the terminal of a called subscriber. The terminal has a ringing tone signal detecting circuit for receiving and detecting the tone signal from the subscriber line.
A digital subscriber line transmission system according to North American Standards has a transmission rate of 80 kHz and employs, as a transmission code, a 2B1Q code composed of foyer values of .+-.1, .+-.3. The ringing tone signal comprises successive .+-.3 signals having a frequency of 10 kHz, as indicated as a received signal (tone signal) a in FIG. 5 of the accompanying drawings. More specifically, a +3 signal is transmitted four times at a trans, mission rate of 80 kHz, and thereafter a -3 signal is transmitted four times. Such a signal transmission cycle is repeated ten times for 3 msec., thereby producing a tone signal.
FIGS. 2(a) and 2(b) of the accompanying drawings show simulated tone signal waveforms that are received. In FIG. 2(a), the tone signal waveform is generated when the subscriber line is 0 m long, i.e., when the subscribers are directly coupled. In FIG. 2(b), the tone signal waveform is generated when the subscriber line is 7 km long and has a diameter of 0.5 mm. It can be seen from FIGS. 2(a) and 2(b) that though the illustrated tone signal waveforms have different voltage levels, they are similar to sine wave-forms.
FIG. 1 of the accompanying drawings shows a conventional ringing tone signal detecting circuit. The conventional ringing tone signal detecting circuit, generally denoted at 3, has a bandpass filter 4, a full-wave rectifier 5, a low-pass filter 6, and a comparator 7.
An analog signal 11 that has been propagated through subscriber line 1 and received by a hybrid circuit 2 is supplied to the bandpass filter 4. Since a 10 kHz signal is used as a tone signal in an echo-canceler digital subscriber line transmission system, the bandpass filter 4 has a central frequency of 10 kHz. A signal component of 10 kHz that is detected by the bandpass filter 4 is then applied to the full-wave rectifier 5 which rectifies the signal. The rectified signal is then converted into a DC signal component by the low-pass filter 6. The DC signal component detected by the low-pass filter 6 is applied to the comparator 7. The comparator 7 compares the DC signal component with a predetermined threshold voltage to determine whether the received signal is a tone signal or not. The conventional ringing tone signal detecting circuit 3 shown in FIG. 1 is of the power detection type because it determines whether the received signal is a tone signal or not based one the power of the received signal.
Since the power of the 10 kHz signal component contained in the received analog signal is extracted only by the bandpass filter 4, any signals other than the tone signal are erroneously detected as the tone signal insofar as they contain the 10 kHz signal component which is of the same frequency as that of the tone signal. For example, if a noise signal as shown in FIG. 3 of the accompanying drawing is applied as a received signal upon a lightning surge, the signal is mistaken as the tone signal due to the 10 kHz signal component contained in the signal. Furthermore, ordinary received signals may sometimes be detected as the tone signal.
The conventional ringing tone signal detecting circuit 3 of the power detection type tends to erroneously detect a near-end crosstalk as the tone signal because the power of the tone signal is attenuated, especially if the transmission length is long.
The conventional ringing tone signal detecting circuit 3 has another problem in that the bandpass filter 4, the full-wave rectifier 5, and the low-pass filter 6, which are in the form of analog circuits, suffer large characteristic charges owing to aging, temperature variations, and manufacturing errors.